A transient blocking unit (TBU) is an arrangement of two or more transistors connected to each other in series such that they automatically switch off when a TBU current passing through these transistors exceeds a predetermined threshold. As the TBU current increases, voltage drops across the TBU transistors are provided as gate voltages to the TBU transistors that tend to turn these transistors off. When the TBU current is below threshold, this turning-off effect is negligible, and the TBU resistance is low. When the TBU current goes above threshold, the turning-off effect is strong enough to cause positive feedback, which leads to rapid and automatic switching of the TBU to a high-resistance off state. The positive feedback in this situation arises from the tendency of an increasing gate voltage on a TBU transistor to increase the voltage drop across that TBU transistor, and this increased voltage drop is provided as a gate voltage to another TBU transistor. The TBU circuit connections are such that positive feedback can result from these increasing transistor and gate voltages.
FIG. 1 shows a simple example of a conventional TBU. In this example, two depletion mode (i.e., normally-on) transistors, Q1 and Q3, are connected in series such that the same current ITBU flows through Q1 and Q3. As ITBU increases, VDS of Q1 and VSD of Q3 both increase. The transistor types are selected such that as VDS of Q1 increases, the voltage applied to the gate of Q3 acts to shut off Q3. Similarly, as VSD of Q3 increases, the voltage applied to the gate of Q1 acts to shut off Q1. The positive feedback inherent in this arrangement leads to a rapid transition of the TBU from a normal low-impedance state to a high-impedance current blocking state once ITBU exceeds a predetermined threshold. In operation, a TBU can switch to its high-impedance state in response to an over-current condition from an electrical transient, thereby protecting electrical devices or circuits connected to the TBU.
The TBU example of FIG. 1 is a unipolar (or uni-directional) TBU because it is only effective to block surges having a predetermined polarity (i.e., either positive surges or negative surges). FIG. 2 shows a conventional bipolar TBU. The circuit of FIG. 2 can be understood as providing two unipolar TBUs having opposite polarity in series. The first unipolar TBU is formed by the combination of Q1 and Q3, and the second unipolar TBU is formed by the combination of Q2 and Q3. FIG. 2 also shows a typical application for a TBU, where it is placed in series between an electrical device 202 and a load 204 in order to protect device 202 from transients from load 204. Protection of device 202 is also provided from transients that may develop between the TBU and load 204 (e.g., on a transmission line). If a transient from the load causes the TBU current to exceed the TBU threshold, the TBU will switch to its off state, thereby protecting the device from the transient. Some early examples of TBUs include U.S. Pat. No. 3,916,220, U.S. Pat. No. 4,533,970, and U.S. Pat. No. 5,742,463.
However, in some applications, the requirement for the TBU current to reach its threshold before the TBU turns off may pose a problem. For example, the protected device may not be able to sink the TBU threshold current. Although one would ordinarily match the TBU to its protected device such that the protected device can sink the TBU threshold current, the protected device may have multiple modes, each mode having a different current handling capacity. For example, the protected device could have a normal operating mode, where it can sink the TBU threshold current, and it could also have a standby or sleep mode, where it cannot sink the TBU threshold current. In this example, if a load transient occurs while the protected device is in its standby mode, an undesirable over-voltage situation could occur at the protected device, because the TBU is not able to switch to its high impedance state.
Accordingly, it would be an advance in the art to provide a transient blocking unit capable of switching off in some situations where the TBU current is below threshold.